Apparatus reading and recording information such as players, recorders and drives for an optical information recording media, for example, MO, CD, or DVD, include an optical pick up device. The optical pick up devices have an optical unit which exposes light having predetermined wavelength to the medium and accepts the reflected light by a detecting element, and the optical units have an optical element such as an objective lens which condensing the light flux at a reflective layer of the recording media or accepting elements. Plastic materials are suitably used for the optical elements of the optical pickup lens in view of low cost production by such as injection molding. Copolymers composed of cyclic olefin and α-cyclic olefin are known for the suitably applicable the optical element. (For example, Patent Document 1)
Blu-Ray Disc (“BD”) recording apparatus or media recording or reproducing information employs an objective lens having a numerical aperture NA of image side of 0.80 to 0.90, representatively 0.85, and light source having wave length of 380 to 420 nm (usually 405 nm) have been developed which can record information with higher density than CD (NA: 0.45, wavelength: 780 nm) or DVD (NA: 0.65, wavelength: 635 or 650 nm).
Patent Document 1: JP A 2002-105131
A spot beam can be made small in the optical pickup apparatus for BD use shorter wave length and having larger NA than the optical pickup lens for conventional CD or DVD, and realize high recording density. On the other point of view, high sensitivity and strict optical performance are required. Light stability different from the conventional optical pickup lens is required adapted for shorter wave length of light source, and higher level of stability performance used at high temperature is required, and therefore, it is not responsible to proper stability against light or heat of the resin material. Very high accuracy as well as high stability are required because it is exposed to strict temperature condition due to heat of driving device such as tracking in addition to high stability against light, particularly for an objective lens which condense the light flux to the recording media. The inventors have found that the deterioration of lens performance is caused by stress relaxation at high temperature and resin deterioration due to blue laser of wavelength of 280 to 420 nm in complex as a result of study for stability performance against light and durability performance.
In view of foregoing, an object of the present invention is to provide a production method of an objective lens used for an optical pickup apparatus, an objective lens used for an optical pickup apparatus and an optical pickup apparatus in which the durability and deterioration of aberration are improved.